Lightning component to render a rich text field unescaped

Did you try

<ui:outputRichText aura:id="outputRT" value="{!v.chatterPost.Body}" />

Which would result in the proper output

I believe they way you are doing it, is the final output of the ui component but with the escaped version of the value.

You could also do

<div class="uiOutputRichText" linkify="true" >
     <aura:unescapedHtml value="{!v.chatterPost.Body}"/>
</div>

To output pre-formatted HTML, use aura:unescapedHTML. For example, this is useful if you want to display HTML that is generated on the server and add it to the DOM. You must escape any HTML if necessary or your app might be exposed to security vulnerabilities.

https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/components_html.htm